To sense air leak with simple operation by calculating an air leak quantity based on time for pressure to rise to a predetermined pressure value when compressed air is supplied and time for the pressure to drop to a predetermined value when air supply is stopped.

Air exits of all terminal devices are closed, a valve 4 is opened, compressed air A at a predetermined air dispense quantity Q is supplied into a pipe line 2 by a compressor 1, pressure rise time T1 since air pressure in the pipe line 2 reaches a set air pressure value P1 of a pressure sensor 11a until a set air pressure P2(set higher than the pressure value P1) of a pressure sensor 11b is reached is measured 12a, and supply of the air A is stopped after the air pressure exceeds the pressure value P2. When air leaks in a pipe line 3, the air pressure gradually drops as supply of the air A stops, and pressure drop time T2, until the air pressure reaches from the pressure value P2 to the pressure value P1 is calculated 12b. A calculator 13 substitutes the pressure rise and drop times T1, T2 and the air dispense quantity Q into an equation of U= Q.T1/T2 to obtain an air leak quantity U.